1. Field of the Invention
This invention relates in general to track following servo control for data storage systems, and more particularly to a method and apparatus for compensating for lateral media shift resulting in forward to backward offset in tape guiding.
2. Description of Related Art
Due to the Internet and the globalization of manufacturing and service-oriented industries, businesses are collecting and analyzing data at an ever increasing rate. As organizations' data warehouses grow, the threat of data loss carries a potentially larger fiscal price. For example, the value of data often exceeds the value of the computer system within weeks of installation of the system. Many factors can cause the loss of data including power brownout, power failure, human error, hard disk failure and other natural disasters. In many cases, the lost data, if not properly backed-up, is simply not recoverable. In other cases, data may still be re-assembled or re-captured from other system either electronically or manually, but both certainly involve a lot of cost and time.
As computing environments have evolved toward data-intensive networks of servers with high system administration costs, the techniques for reliable backup have had to change to meet new needs. The art of data backup has evolved significantly in recent years from individual SCSI drives backing up single computers to automated library systems backing up heterogeneous networks of servers.
Data storage on magnetic tape is well known and tape recorders have been used to record data tracks on magnetic tape. Traditionally, data is recorded in a plurality of parallel data tracks on the magnetic tape. The head is then positioned relative to the tape path by moving the read/write head to different track positions as desired while holding the read/write head stationary. In such a system, the tape tracks must both be sufficiently wide and separated to guarantee that the exposure of the data track to the head is accurate at least to the minimum requirements necessary to reliably read and write the data. The read/write head is positioned at a predetermined fixed point, relative to the magnetic tape path and the data track must accommodate variations of recording track location and tape location variances as the tape feeds past the head. Historically, this accommodation has been accomplished insuring that the track width and the data track separation on the magnetic tape are sufficient for the read/write head to remain positioned over the designated track and at the same time not read magnetically recorded signals from an adjacent track. This arrangement of track width and track separation will accommodate any deviation of the track location from the design norm either due to being recorded on a first recorder and played or rerecorded on a second recorder or due to the wander of the tape as it is spooled past the read/write head from one spool to the other spool of a cartridge.
The definition of the magnetic read/write head and the track width and separations effectively limits the number of data tracks that may be recorded on any given width of tape. Reliance solely on the track width and track separations to insure reliable read/write operations results in a significant waste of magnetic tape surface and thus limits the data density on the tape. Data may be recorded in tracks that are much narrower and still be reliable from a read/write standpoint, but the read/write head must be and remain perfectly aligned with the data track. However, as the track width and the read/write head width narrow in an effort to increase the data capacity of a given tape area, any misalignment of the head with the track may lead to read/write repeatability failures and lost data. Thus, the resolution of the head placement mechanism and the precision of the placement of the tape relative to the read/write head become limiting factors affecting the recording density of data on the magnetic tape surface in tape drives having static read/write heads.
In high track density tape storage devices, a compound actuator is used for track following system. The compound actuator includes a coarse actuator which usually is a stepper motor and a fine actuator which has a linear high bandwidth, limited range of travel. The compound actuator thus has the advantage of high bandwidth and large working dynamic range. Another important aspect of the tape drive is the tape guide which guides the tape over the read/write/servo head in a stable manner to allow reliable track following.
Typically, tape drive systems provide tape guides for controlling the lateral movement of the tape as the tape is moved along a tape path in a longitudinal direction across a tape head. The tape may have a plurality of data tracks extending in the longitudinal direction. More recently, tape drive systems have used a track following servo system for moving the tape head in a lateral direction for following lateral movement of the longitudinal tracks as the tape is moved in the longitudinal direction. The track following servo system may employ servo tracks on the tape which are parallel to the data tracks, and employ servo read heads to read the servo tracks to detect position error and thereby position the tape head at the data tracks and follow the data tracks. This allows the data tracks to be placed closely together and increase the number of data tracks.
The tape is typically contained in a cartridge of one or two reels, and the tape is moved between a supply reel and a take up reel. The reels typically have runout causing the tape to move laterally as the tape is moved longitudinally. Tape guides provide the conventional means for limiting at least the amplitude of the lateral movement of the tape so that it does not exceed the lateral movement capability of the track following servo system.
Typical tape guides may comprise stationary buttons or edges, or flanges at the side of tape guide rollers, positioned against the edges of the tape to control the amplitude of the lateral movement of the tape. In order to increase the total capacity of a tape, the tape is increasingly made thinner to allow more wraps of tape to fit on a given tape reel. As a result, the tape is very weak in the lateral direction, and can easily be damaged at the edge from the tape guide. Thus, the tape guides are typically positioned at a bearing where the tape assumes a cylindrical shape, thus increasing the tape edge ability to support a load. The bearing is also typically designed to have low friction. This arrangement minimizes the potential to distort the edge of the tape as the guides push against the edges of the tape to move the tape to the center of the bearing to reduce the amplitude of lateral displacement of the tape. One example is illustrated in U.S. Pat. No. 5,447,279, which employs an air bearing to reduce the friction of the bearing for stationary tape guides. Roller bearings may also be utilized for reducing the friction of the bearing while the flanges of the roller bearings push against the edges of the tape. One example of a roller bearing or fixed pin with flanges arranged to have low friction is U.S. Pat. No. 4,427,166. Fixed surfaces may also be arranged to have low friction. One example is described in U.S. Pat. No. 4,466,582, where a synthetic resin or metal coated tape guide bearing has a reduced contact area for the tape to lower the friction between the guide surface and the running tape and allow the flanges to stabilize the tape.
However, when wound on a reel, tape is typically subjected to stack shifts or stagger wraps, in which one wrap of the tape is substantially offset with respect to an adjacent wrap. Thus, as the tape is unwound from the reel, there is a rapid lateral transient shift of the tape. Other common sources of rapid lateral transient shifts include 1) a buckled tape edge in which the tape crawls against a tape guide flange and suddenly shifts laterally back down onto the bearing, 2) a damaged edge of the tape which causes the tape to jump laterally when contacting a tape guide, and 3) when the take up reel or supply reel runout is so significant that the reel flange hits the edge of the tape.
As mentioned above, a servo read/write head positioning drive is now typically incorporated into the tape drive system in order to position the head precisely relative to a moving data track on tape. To provide the locational control of the servo read/write head positioning drive, a magnetic read head gap may be placed at a position relative to a servo track on the tape. Then the read head gap is further moved to detect the edge of the servo control track (servo track) recorded on the tape. The read head will provide signals which may be used to indicate the head location relative to the servo track. By using these signals as a basis, the servo control then may produce a positioning signal to drive a servo positioner. The servo positioner moves the read head, causing the read head to track or follow the edge of the servo control track which has been previously recorded on the tape.
The transient response of the tape head track following servo system typically comprises a high bandwidth for a very limited lateral movement, called “fine” track following, for allowing the tape head to accurately follow small displacements of the tape. Larger movement of the tape head is typically conducted as “coarse” track following, which is also employed to shift the tape head from one set of tracks to another set, and is conducted at a slow rate. The occurrence of a lateral transient shift, however, is so rapid that neither the fine track follower nor the coarse track follower is able to respond, with the result that the tracking error becomes so large that writing must be stopped to prevent overwriting an adjacent track and to insure that the tracking error on read back is not so large as to cause a readback error.
One approach has been to make the tape guide edges or flanges closer together to maintain a pressure on both edges of the tape. However, this tends to stress and damage the edges of the tape, reducing its durability. An attempt at reducing the stress comprises spring loaded tape guides, such as the above-mentioned '279 patent. However, although the amplitude of the tape shift may be reduced somewhat by this approach, the speed of the shift is typically not reduced, and the track following servo error still occurs, reducing the performance of the tape drive.
Various attempts have been made to compensate for tape shifts. For example, a tape movement constraint is provided for a tape drive system which moves a tape longitudinally along a tape path across a tape head, the tape having longitudinal tracks and a tape roller bearing is positioned closely adjacent the tape head. The tape roller is rotatable about a central axis parallel to the cylindrical peripheral surface, allowing the tape freedom of movement in the longitudinal direction. The tape roller bearing has a frictional cylindrical peripheral surface for contacting and engaging the surface of the tape and constraining movement of the tape in the lateral direction, thereby reducing the rate of the lateral transient movement of the tape to allow the track following servo system to follow the reduced rate lateral transient movement of the longitudinal tracks. Any potential air bearing that could form between the surface of the tape and the surface of the roller bearing is collapsed to insure that the roller bearing frictionally contacts and engages the surface of the tape.
Nevertheless, a problem is associated with a tape drive that uses a spiral grooved roller tape guide for controlling lateral tape motion transients in the tape path. The grooved roller has the tendency to bias the media toward the direction of forward spiral. This creates offsets in the tape guiding position that depend on the direction of tape motion. For example the tape is biased down to the bottom flange when the guide is rolling clockwise and biased to the top flange when the guide is rolling counter clockwise. This biasing of tape results in relatively large lateral displacement between forward and backward direction and in the tape drive. This movement could shift the tape servo format completely off of the servo head element resulting in loss of servo signal and ultimately failure in acquiring track lock.
It can be seen that there is a need for a method and apparatus for compensating for media shift due to tape guide.